There currently exist numerous uses for apparatus that treat a specimen or workpiece at high pressures and high temperature including, for example, gas pressure bonding furnaces and hot isostatic pressing apparatus. In these apparatus, it is typical to treat a workpiece at 1000.degree. C. and 15,000 psi although these are not the maximum temperature and pressure conditions encountered. Suitable apparatus for these applications generally comprise a furnace within a pressure vessel or autoclave. The furnace provides the heat to the workpiece and protects the vessel from excessive temperature. The vessel maintains the furnace and the workpiece at the desired pressures.
In most processes, it is essential that the temperature of the workpiece be extremely uniform. Otherwise, problems may result from differential thermal expansion of the workpiece. Thus, the furnace portion of the high pressure-high temperature apparatus must distribute the heat evenly to the workpiece. There exist a number of applications of high temperature-high pressure vessels as described herein, where it is desirable to cool the workpiece within the workspace as fast as possible after treatment is completed. For example, certain metallic alloys after being heat treated in the vessel will have a finer grain size if more rapidly cooled from say 1250.degree. C. to 750.degree. C. This will improve the performance of the alloys in certain applications. More rapid cooling is also desirable for increasing the turn around of the vessel, i.e., the number of heating and cooling cycles in a given period of time. Until now, there have been two suggested approaches to more rapid cooling of high temperature-high pressure vessels. The circulation of the pressurized gases within the vessel is promoted and/or hot pressurized gases are with the aid of a pump withdrawn from the vessel, cooled in a heat exchanger and returned to the vessel. These basic concepts are dealt with in our U.S. Pat. No. 4,022,446. It has also been suggested to circulate cooling liquids to a heat exchanger within the pressure vessel and mechanically circulate the atmosphere within the vessel as shown in U.S. Pat. No. 3,168,607.
Applicants have now developed techniques for promoting rapid cooling of the pressurized furnace making use of the transfer of the mass of pressurized gases within the vessel to the exterior of the vessel as required for depressurizing the vessel. In a preferred embodiment, the Joule-Thomson effect is utilized to increase the effectiveness of the heat transferred from the vessel by the escaping gases.
The Joule-Thomson effect is a phenomenon resulting in a difference in temperature between compressed and released gas passing at high pressure through a porous plug or small aperture referred to herein as a Joule-Thomson valve. The equations describing this effect contain two partial derivatives: ##EQU1## The expression on the left is the rate of change of temperature with pressure at constant heat content. The expression on the right has in its numerator the difference between the product of the temperature and the rate of change of volume with temperature at constant pressure, from which the volume is subtracted. The denominator contains the molar specific heat at constant pressure. The term on the left of the equality sign is called the Joule-Thomson Coefficient. It varies with the temperature and pressure of the gas, passing from positive values through zero to negative values. The temperature at which it is zero is called the Joule-Thomson Inversion Temperature (hereafter the inversion temperature) and varies with the particular gas. For a van der Waals gas, the equation becomes: ##EQU2## The term in parentheses on the right side in the preceding equation can be positive or negative. It is apparent that the inversion temperature at which the sign of dT/dp changes is that for which 2a/RT.sub.i =b or T.sub.i =2a/bR.